The present invention relates in general to an extruded film processing system, and in particular to control systems utilized in extruded film processing systems.
Blown film extrusion lines are used to manufacture plastic bags and plastic sheets. A molten tube of plastic is extruded from an annular die, and then stretched and expanded to a larger diameter and a reduced radial thickness by the action of overhead nip rollers and internal air pressure. Typically, ambient air is entrained by one or more blowers. The ambient air provides a cooling medium, which absorbs heat from the molten material. This speeds up the change in state from a molten material back to a solid material.
Additionally, the ambient air entrained by the blowers is used to provide air pressure, which is utilized to control the size and thickness of the film tube. One type of blown film extrusion line utilizes air flow on the exterior surface of the film tube in order to absorb heat. A different, and more modern, type of blown film extrusion line utilizes both an external flow of cooling air and an internal flow of cooling air in order to cool and size the film tube. Whether the blown film tube is cooled from the interior surface, the exterior surface, or some combination of the two, one common problem in blown film extrusion lines is that of obtaining precise control over the diameter of the extruded film tube. Tight control over the diameter ensures uniform product dimensions, which includes the size of the extruded product, as well as the thickness of the plastic material.
Acoustic sensors may be utilized to gauge the diameter of the product. When such acoustic sensors are utilized, a feedback loop is established to alter dynamically one or more controllable variable of the process, such as blower speed, and/or temperature control over the cooling air stream.
It is one objective of the present invention to provide a substantially improved ability to keep blown film product width within established specifications. This invention provides improved lay-flat control by adding a second feedback control loop, in addition to, and or supplementation of, the primary control feedback loop which is utilized to control the extrusion and cooling process.
This additional and/or supplemental control loop of the present invention measures actual bubble diameter, preferably (but not necessarily) utilizing acoustic sensors, and feeds back this information to one or more controllers. Preferably the controller is the one which is utilized to perform the calculations and control operations of the primary control loop for expanding and cooling the extruded film tube. The sensed diameter data is compared against an operator established set point. In the preferred embodiment, the resulting error is injected into the Internal Bubble Cooling system (the xe2x80x9cIBCxe2x80x9d) to provide a correction effect. In the preferred embodiment, this is in fact directly added as an input to the primary control loop.
Preferably one or more non-contact acoustic sensors are located above the so-called xe2x80x9cfrost linexe2x80x9d, thus providing a measure of the diameter of the product after cooling but preferably BEFORE flattening of the extruded film tube by an assembly of collapsing boards and nip rollers. In most conventional blown film lines, this assembly is located overhead of the die and related components. Thus the diameter sensors of the present invention are located above the sensors of the primary control loop for controlling product diameter (through control of the expansion and cooling of the extruded film tube) but beneath the collapsing boards and nip rollers. This preferred placement of the second set of bubble diameter measuring devices of the present invention above the IBC sensors provides a quicker response than established methods in the prior art. A variety of alternative sensors may be utilized in lieu of an acoustic sensor. For example, mechanical feeler arms may be utilized, especially if the sensor is located sufficiently far from the frost line to minimize the chance of creating deformations in the product through contact with the mechanical feeler arms. As a particular matter, an acoustic sensor works fine since it has no moving parts and creates no pressure on the tube or bubble. It may however be difficult (but not impossible) to use optical sensor since the sensor response would be dependent on the color of the extruded tube. Accordingly, the preferred sensor is any non-optical sensor.
The prior art approach is characterized by the utilization of a lay-flat measuring bar after the primary nip rollers. In the prior art systems, the distance between the IBC sensors (of the primary control loop) and the lay-flat bar can be nearly 40 feet and when oscillating nip devices are used; of course, this path length of the prior art approach can vary as the nip oscillates.
One additional problem of the prior art is resolved by the present invention. IBC performance depends on stable airflow sources to maintain a stable bubble. Therefore, disturbances can result in changes in the final product width. In particular, rotating or oscillating dies use moving air chambers that can induce a disturbance in the airflow as a result of uneven airflow in the chamber. In the present invention, the variation in product diameter resulting from the airflow changes that occur because of imbalances in the rotating chamber can be significantly reduced.
In accordance with the preferred embodiment of the present invention, one or more sensors are positioned in a different horizontal plane from the IBC control sensors. Preferably, these sensors are also placed in a different circumferential position than the primary control loop sensors. In this patent, these sensors are called xe2x80x9clay-flatxe2x80x9d sensors to distinguish them from the IBC sensors. In the preferred embodiment, the placing the lay-flat sensors in a horizontal plane vertically above the IBC sensors provides optimum results. The purpose of these sensors is to provide a measurement of the actual bubble diameter from which the final lay-flat dimension can be calculated from a simple formula (lay-flat equals pi multiplied by the sensed diameter divided by two).
The preferred system of the present invention monitors the sensor(s) for proper operation and selects which particular sensors are allowed to contribute to the bubble diameter measurement. It also provides an indicator when all sensors are not allowed to contribute. The system filters the received signal from one or more sensors and calculates the expected lay-flat.
This system can also accept a calibration input from the operator. This calibration input allows the operator to indicate the current actual lay-flat as measured at the point of accumulation (such as a spooling system) for the material. The system takes this reading and back calculates an adjustment factor that accounts for the xe2x80x9cdraw downxe2x80x9d of the material.
Draw down is the amount the material shrinks in width as a result of the tension placed on the material during accumulation. The amount of draw down is dependent upon both the material utilized in the extrusion line and the amount of tension utilized in the accumulation operations. Thus the amount of xe2x80x9cdraw downxe2x80x9d is a function of both material and tension. The mixture and composition of the material input into the blown film line is relatively fixed for each product run; however, the material can vary greatly in composition (and associated physical properties) between product runs. The amount of tension applied to the accumulation or spooling system also varies between production lines and production runs; however, the amount of tension applied is susceptible to a greater amount or range of operator (and computer-system) control.
Accordingly the lay-flat feature of the present invention is useful over a wide variety of materials, which are used in blown film line, and it is also useful over a wide range of production equipment.
In accordance with the preferred embodiment of the present invention, the system converts the actual lay-flat signal into a signal that matches the signal type used by the IBC sensor; in other words, the lay-flat signal can be translated to the units and scale utilized by the primary control loop. The system directly accepts as an input the converted lay-flat signal and compares it to the operator-established set point.
The system also monitors the signal rate of change and position against operator set windows of operation. This system essentially decides if the lay-flat signal is stable and within acceptable range for proper corrective action. If the signal is acceptable, the system applies an adjustable gain, inverts the signal and injects the signal into the IBC control system.
The above as well as additional objectives, features, and advantages will become apparent in the following description.